Abstract

Multi-channel seismic profiles from the Canterbury Basin on the eastern margin of the South Island of New Zealand reveal the importance of current activity in shaping a Neogene shelf sediment prism. The shelf prism prograded across a broad, near-horizontal platform in water depths of 1,000 to 1,750 m. The platform was formed above a condensed section of late Eocene to late Oligocene limestones which overlie Cretaceous to Paleogene rift-fill and transgressive sediments.

The Neogene sediment prism contains sediment drifts which are as much as 25 km long and 15 km wide and extend up to 1,600 m (uncompacted) vertically. Individual drifts migrated westward and can be traced between dip profiles, revealing that the long axes of most are subparallel to the present coastline and shelf-edge. Channel-like features at the landward edges of the drifts correspond to residual space left between the landward-prograding off-shelf sediment drift and the adjacent shelf foreslope. Erosion or slow deposition characterized the foreslope. Progradation of the shelf was by the accretion of successive sediment drifts. Before ca. 11.5 Ma (= Pink Horizon), the shelf-edge-parallel drifts were distributed across the central part of the basin, whereas subsequently they were concentrated to the northeast.

The seismic architecture of the Neogene sediment prism results from the interplay of an abundant western sediment source and an offshore boundary current system. Present-day ocean circulation involves northward flow along the east coast of the South Island. The basin may have been subjected to a middle Miocene to late Pliocene phase of intensified flow, caused by local topographic enhancement and/or global paleoceanographic events. Current activity has played a crucial role in the sedimentary evolution of the Canterbury Basin Neogene shelf prism.

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